Over the past several years, there has been considerable interest in using optical scatterometry (i.e., optical diffraction) to perform measurements associated with semiconductor fabrication. One area of great interest has been the critical dimension (CD) measurements of the lines and structures included in integrated circuits. Optical scatterometry has been used to analyze periodic two-dimensional structures (e.g., line gratings) as well as three-dimensional structures (e.g., patterns of vias or mesas). Scatterometry is also used to perform overlay registration measurements. Overlay measurements attempt to measure the degree of alignment between successive lithographic mask layers. (See, U.S. 2002/0158193, incorporated by reference) Scatterometry measurements have also been proposed for monitoring etching, dishing, planarity of a polished layer, control of gate electrode profiles, film stack fault detection, stepper control, deposition process control and resist thickness control. See for example, U.S. Pat. Nos. 6,464,563; 6,451,700; 6,433,871; 6,458,610; 6,479,200; 6,383,824; 6,458,605 and 6,451,621, incorporated herein by reference. The assignee herein has also proposed to use scatterometry analysis to directly measure ion implantation structures. (See, U.S. patent application Ser. No. 10/339,147, filed Jan. 9, 2003-TWI 21800, incorporated herein by reference).
Various optical techniques have been used to perform optical scatterometry. These techniques include broadband scatterometry (U.S. Pat. Nos. 5,607,800; 5,867,276 and 5,963,329), spectral ellipsometry (U.S. Pat. No. 5,739,909) as well as spectral and single-wavelength beam profile reflectance and beam profile ellipsometry (U.S. Pat. No. 6,429,943). In addition it may be possible to employ single-wavelength laser BPR or BPE to obtain CD measurements on isolated lines or isolated vias and mesas (see, U.S. patent application Ser. No. 10/243,245, filed Sep. 13, 2002). Each of these documents are incorporated herein by reference.
Each of these prior techniques use a non-intensity modulated probe beam and the DC scattering is measured. The prior art also includes discussions of measuring light scattered from a sample that has been periodically excited. One such disclosure appears in U.S. Pat. No. 4,632,561 assigned to the same assignee as herein. In the system disclosed in the latter patent, scattered light was used to evaluate subsurface features of the sample such as material composition. The apparatus included an annular detector which was configured to measure the light scattered in all directions. The total amount of detected light was used as a direct measure of material composition. It should be understood that in this system, no effort was made to evaluate the sample based on how the physical structure diffracted light into different orders. Rather, the effort was merely to evaluate the overall change of reflectivity of the sample as expressed in the scattered light signal.
In contrast, optical “scatterometry” attempts to evaluate the geometry of a sample based on the pattern of the diffracted light. More specifically, scatterometry systems use a modeling approach to transform scatterometry measurements into geometric measurements. For this type of approach, a theoretical model is defined for each physical structure that will be analyzed. The theoretical model predicts the empirical measurements (scatterometry signals) that scatterometry systems would record for the structure. A rigorous coupled wave theory can be used for this calculation. The theoretical results of this calculation are then compared to the measured data (typically in normalized form). To the extent the results do not match, the theoretical model is modified and the theoretical data is calculated once again and compared to the empirical measurements. This process is repeated iteratively until the correspondence between the calculated theoretical data and the empirical measurements reaches an acceptable level of fitness. At this point, the characteristics of the theoretical model and the physical structure should be very similar.
Evaluation of the theoretical models is a complex task, even for relatively simple structures. As the models become more complex (particularly as the profiles of the walls of the features become more complex) the calculations can become extremely time consuming. Even with high-speed processors, real-time evaluation of these calculations can be difficult. Analysis on a real-time basis is very desirable so that manufacturers can immediately determine when a process is not operating correctly. The need is becoming more acute as the industry moves towards integrated metrology solutions wherein the metrology hardware is integrated directly with the process hardware.
A number of approaches have been developed to overcome the calculation bottleneck associated with the analysis of scatterometry results. Many of these approaches have involved techniques for improving calculation throughput, such as parallel processing techniques. An approach of this type is described in a co-pending application PCT WO 03/009063 (incorporated by reference) which describes distribution of scatterometry calculations among a group of parallel processors.
Another approach is to use pre-computed libraries of predicted measurements. This type of approach is discussed in U.S. Pat. No. 6,483,580 (Xu), incorporated herein by reference. In this approach, the theoretical model is parameterized to allow the characteristics of the physical structure to be varied. The parameters are varied over a predetermined range and the theoretical result for each variation to the physical structure is calculated to define a library of solutions. When the empirical measurements are obtained, the library is searched to find the best fit.
In a variation on this approach, the library data is used as a starting point and an estimation or interpolation algorithm is used to refine the results. U.S. Pat. No. 5,867,276 describes a system of training a library to permit linear estimations of solutions. Another form of interpolation can be found in U.S. patent application Ser. No. 2002/0038196, published Mar. 28, 2002. This use of interpolation avoids the penalty associated with generating results in real-time, but may sacrifice accuracy during the interpolation process. Any of these approaches could be used with the subject invention.